1. Field of the Invention
The present invention relates to a thermal head and a method for manufacturing it and, more particularly, to a thermal head suitable for high speed printing which has high heat resisting characteristics and good thermal response and a method for manufacturing the same.
2. Prior Art
Thermal heads incorporated in thermal printers are generally used for recording, for example, by linearly arranging a plurality of heating resistors on a same substrate and by energizing and heating the heating resistors in accordance with desired printing information to cause a thermosensible recording paper to develop a color or by transferring ink onto a plain paper through an ink ribbon.
FIG. 5 shows a conventional thermal head wherein a glazed layer 2 made of glass or the like acting as a heat accumulating layer is formed on an insulating substrate 1 made of ceramics such as alumina. The glazed layer 2 is formed so that its upper surface will have a circular section. A plurality of heating resistors 3 made of Ta.sub.2 N (tantalum nitride) or the like are coated on the upper surface of the glazed layer 2 through vapor deposition, sputtering or the like, etched, and linearly aligned in accordance with the number of dots. On one side of each heating resistors 3, a common electrode 4 which is connected to each heating element 3 is formed. On the other side, an individual electrode 5 for individually energizing respective heating resistors 3 is connected. The common electrode 4 and the individual electrode 5 are made of aluminum, copper, gold or the like. for example, and are formed by coating through vapor deposition, sputtering or the like and by patterning into desired shapes through etching.
Further, on the surfaces of the heating resistors 3, common electrodes 4 and individual electrodes 5, a protective layer 6 having a thickness of approximately 5 to 10 .mu.m is formed to protect the heating resistors 3, common electrodes 4 and individual electrodes 5. The protective layer 6 is formed so that it covers the entire surfaces except terminal portions of the electrodes 4 and 5. The protective layer 6 has a construction wherein an oxidation resistant layer 7 made of SiO.sub.2 or the like having a thickness of approximately 2 .mu.m for protecting the heating resistors 3 against deterioration due to oxidation and an abrasion resistant layer 8 made of Ta.sub.2 O.sub.5 or the like having a thickness of approximately 3 to 8 .mu.m for protecting the heating resistors 3 and electrodes 4 and 5 against abrasion caused by contact with an ink ribbon or a thermosensible paper are laminated in this order. The oxidation resistant layer 7 and the abrasion resistant layer 8 are sequentially formed by means of vapor deposition, sputtering or the like.
In a thermal printer utilizing such a thermal head, printing is carried out as desired by selectively energizing the individual electrodes 5 of the heating resistors 3 in accordance with a predetermined printing signal, with the thermal head pressed into contact with a paper transported onto a platen through an ink ribbon or directly in the case of a thermosensible recording paper, to cause desired heating resistors to generate heat, thereby fusing ink on the ink ribbon and transferring it onto the paper or causing the thermosensible recording paper to develop a color.
In such a thermal head, balance is maintained between power efficiency and printing characteristics making use of heat accumulating effect of Joule heat generated by the heating resistors 3 through the combination of the glazed layer 2 of low heat conductivity (2.times.10.sup.-3 cal/cm.Sec..degree.C.) and the insulating substrate 1 of comparatively high heat conductivity (40.times.10.sup.-3 cal/cm.Sec..degree.C.) made of alumina. Specifically, time constant of cooling of the heat resistors 3 becomes long due to the heat accumulating effect of the glazed layer 2. As a result, there will be deterioration of printing quality such as, smears and blurs on printed letters and stains in blank spaces and missing dots due to overheat of the heating resistors during high speed printing. Therefore, the thickness of the glazed layer 2 is adjusted in accordance with operating conditions taking both power efficiency and printing characteristics into consideration and is normally on the order of 30 to 60 .mu.m.
Increased demands for printers capable of high quality printing with finer printing characteristics at high speeds in recent years has resulted in the introduction of thermal printers having printing resolution of 400 dpi (dot per inch) and a printing speed of 100 cps (character per second) into practical use. In such a thermal printer, energization is controlled with a very small pulse width such that the driving cycle of the heating resistors 3 will be 300 .mu.s or shorter. There is a continuing trend toward finer and faster printing.
Since accumulation of heat at a thermal head has been worsened in such a thermal printer for finer and faster printing resulting in a reduction in printing quality, minute control has been carried out on the temperature rise in the thermal head due to accumulation of heat making the thickness of the glazed layer 2 as small as approximately 30 .mu.m and by correcting the period of energization of the heat resistors 3 by an electrical means utilizing a heat history correcting LSI.
However, when finer and faster printing is performed, it is difficult to prevent the reduction in printing quality due to the accumulation of heat at a thermal head only with such a technique. There is a need for a technique which provides a drastic solution to such a problem of heat accumulation.
Further, it has been thought that the accumulation of heat at a thermal head has been caused only by the glazed layer 2 of low heat conductivity. However, it was revealed that the insulating substrate 1 also constituted a major part of the cause of the accumulation of heat in the case of high speed printing wherein the energizing period of the heating resistors 3 is short as described above.
In addition, when energization is controlled with a very small pulse width such that the driving cycle of the heating resistors 3 will be 300 .mu.s or shorter, predetermined printing energy must be obtained in order to achieve desired printing quality by raising the peak temperature of the heating resistors 3 of the thermal head. For example, if ambient temperature is as low as 5.degree. C. during printing, great energy must be applied to the thermal head to allow printing. This, along with the effect of the accumulation of heat, can raise the peak temperature of the heating resistors 3 to approximately 800.degree. C. which is higher than the temperature the glazed layer 2 can endure which is approximately 700.degree. C. If such a situation occurs, the glazed layer 2 may be thermally deformed or melted, disabling proper printing. Thus, the prior art thermal heads have a problem that they can not be used as printhead for thermal printers to perform finer and faster printing.